Various techniques are available for depositing metals upon a mold. One such technique is single wire electric arc welding. Typically, in single wire electric arc welding, an electric arc is established between the tip of the single wire (i.e., an anode) and a metallic member (i.e., a cathode). The electric arc melts the tip of the wire to form metal droplets. Gas is fed into an arc chamber, usually coaxially to the wire, where it is expanded by the electric arc to form a plasma which comprises a highly heated gas stream carrying a spray of metal droplets from the electrode tip towards the mold. Typically, the current used to create the electric arc is constant throughout the entire electric arc welding process.
It has been well known in the art that the size and the velocity of the metal droplets created by the electric arc are directly proportional to the current employed to create the arc. For instance, high currents result in small droplets having a relatively high velocity, and low currents result in large globular droplets which are transferred to the mold at a relatively low velocity.
Microstructure and mechanical properties of a metal deposit are often affected by the size, velocity and temperature of the metal droplets used to form the deposit. As such there are many instances where it is desirable to produce small droplets, at relatively high velocities (in conventional constant current operation, a particular size of droplet can only have a particular velocity). One such instance is the filling of a narrow deep slot in a mold, where small droplets having a relatively high velocity are more likely to fill the mold without excessive porosity and bridging than is a spray of large droplets with low droplet velocity. In some instances, it may be more desirable to have small droplets having relatively low velocities. One such example might be when depositing the initial layers onto the ceramic mold, when it might be necessary to improve bonding between the metal and the ceramic. There might be cases where it might be necessary to have large droplets at high velocities. Such an example would be to fill the wider areas of the mold or after a few layers have been deposited.
It is also desirable to produce narrow controllable metal sprays having diameters of about 6-9 mm to produce fine features in bulk, net shape or near net shape, spray deposits. Another advantage of a relatively narrow metal spray is a more efficient use of metal as there is less metal being deposited outside of the mold.
Accordingly, it is an object of the present invention to provide a method of controlling the droplet size independently of droplet velocity for a metal spray. It is yet another object of the present invention to provide a relatively narrow metal spray.